1. Field of the Invention
This invention generally relates to wireless communication systems and, more particularly, to a system and method for globally controlling transmitter power levels in a wireless communications network.
2. Description of the Related Art
In a Code Division Multiple Access (CDMA) cellular telephone system, such as that described in Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) Interim Standard 95 (IS-95) entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneously communication between a mobile station and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a pseudonoise (PN) code. The high-speed PN code is used to modulate signals transmitted from the base stations and the mobile stations. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station. The high speed PN modulation also allows the receiving station to receive a signal from a single transmitting station where the signal has traveled over several distinct propagation paths.
The path loss in the CDMA mobile radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the mobile station, operates on a different frequency than the reverse link, which is directed from the mobile station to the base station. However, because the forward link and reverse link frequencies are within the same frequency band, a significant correlation between the average path loss of the two links exists. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time.
In an exemplary CDMA system, each mobile station estimates the total received power of desired CDMA signals on the assigned CDMA frequency channel of the forward link based on the total received power of all signals in the CDMA bandwidth at the input to the mobile station. The total received power is comprised of the sum of the power of a desired CDMA signal received from the base station presently assigned to the mobile station, and the power of various interfering signals that fall within the CDMA bandwidth. Such interfering signals may be received from other CDMA base stations operating on the frequency assigned to the mobile station, as well as from other nearby communication systems. Since the path loss on the forward and reverse links are assumed to be closely correlated, the mobile station uses the estimate of the forward link power to set the transmit level of the reverse link signal. The transmit level of the reverse link signal is adjusted in order to match the estimated path loss on the reverse link, and arrive at the base station at a predetermined level. Such an open-loop power control system is described in U.S. Pat. No. 5,056,109 entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated herein by reference.
Additionally, the mobile station uses the estimated path loss in determining the power level at which it should transmit access probes, in order to establish communications with the base station on the access channel. The access channel provides communications from the mobile station to the base station when the mobile station is not using a traffic channel (i.e. when a call is not already in progress). Access channel messages provide for call originations, responses to pages, orders, and registrations. Since the access channel in a typical CDMA communication system is a random access channel, multiple mobile stations may simultaneously try to use the access channel. Although the mobile station randomly chooses a PN time alignment to minimize collisions with other mobile stations that are simultaneously transmitting on the access channel, each additional mobile station transmitting on the access channel contributes to the background noise on the channel, which has a finite capacity.
Unfortunately, the presence of interfering signals within the total power received by the mobile station tends to significantly reduce the accuracy of the mobile station's estimate of path loss. When such an inappropriately reduced estimate of path loss is used to set the open-loop transmit level of the reverse link signal, the level may be lower than that necessary to ensure the reverse link signal is received with adequate strength at the base station. Similarly, when an inappropriately reduced estimate of path loss results in an overly favorable indication of channel quality being provided to a user, the user may be unsuccessful at initiating communication over the degraded channel. Additionally, the inaccurate estimate of path loss may lead to the mobile station sending out initially weaker access probes, resulting in multiple unsuccessful attempts to establish communication on the access channel, and therefore unnecessarily wasting some of the access channel capacity and having an adverse effect on the balance of system loading. It should be noted that the above-described problems exist in non-cellular CDMA communication systems as well, such as PCS or wireless local loop systems.
Since mobile stations rely upon the measurement of the wideband total received signal power in estimating path loss to the closest cell-site, interfering signals received from other cell-sites and/or from neighboring cellular systems may result in an inaccurately low path loss estimate. As was previously discussed, this may result in an inadequate level of reverse link signal power being transmitted to the closest cell-site, and an overly favorable indication of channel quality being provided to the mobile station user. This inaccuracy may also result in the mobile station being required to transmit more than one access probe to initiate communication on the access channel. These unnecessary multiple probes clutter the system with undesirable interference that result in performance degradation.
The error in the estimate of the quality of the reverse link introduced by the presence of background interference can be eliminated. A solution to this problem is advanced in U.S. Pat. No. 5,799,005, SYSTEM AN METHOD FOR DETERMINING RECEIVED PILOTPOWER AND PATH LOSS IN A CDMA COMUNICATION SYSTEM, Samir Soliman, assigned to the same assignee as the instant application. This patent is incorporated herein by reference. Generally, within the communications receiver, a received signal power of the spread-spectrum signal is measured. A relative pilot strength measurement is also made of the pilot signal received with the spread-spectrum signal. The power of the pilot signal is then computed on the basis of the received signal power and the relative pilot strength measurement.
In another aspect, the patent provides a system and method for estimating path loss of a communication channel between a base station and a remote site station. The base station transmits a pilot signal to the remote site station, and also transmits to the remote site station, an indication of the power at which the pilot signal was transmitted. At the remote site station, a total received signal power over the CDMA bandwidth is measured and a relative pilot strength measurement is made of the received pilot signal. The power of the received pilot signal is then computed on the basis of the received signal power and the relative pilot strength measurement. An estimate of the path loss is then made by determining the difference between the indicated power of the transmitted pilot signal, and the received pilot signal power.
The accurate estimation of path loss can be facilitated through determination of the loss experienced by a spread spectrum “pilot” reference signal. In an exemplary CDMA communication system each base station continuously transmits an unmodulated, direct-sequence spread spectrum pilot signal. This pilot signal allows a mobile station to acquire the timing of the forward CDMA channel, provides a phase reference for coherent demodulation by the mobile station, and provides a means for signal strength comparisons between base stations for determining when to handoff. The pilot signal transmitted by each base station in a CDMA system may use the same PN code, but with a different code phase offset, meaning that the PN codes transmitted by neighboring base stations are identical but skewed in time with respect to one another. Phase offset allows the pilot signals to be distinguished from one another according to the base station from which they originate.
The power of the pilot signal received by a given mobile unit from the base station with which the mobile unit is in communication, is determined using a pair of signal measurements made at the mobile unit. In particular, a pilot strength measurement (in dB) is summed with a measurement of the total signal power (in dBm) received by the mobile unit in order to determine the power of the received pilot signal. Specifically:Ppilot=Ec/I0+Ptotal
where Ptotal is the total received signal power (dBm), Ec/I0 is the pilot strength measurement (dB), and Ppilot is the power of the received pilot signal. The term Ec (energy per chip) corresponds to the received pilot signal energy during one chip period, and the term I0 denotes total received spectral power density in the CDMA signal bandwidth. The pilot strength measurement Ec/I0 is thus seen to provide an indication of received pilot signal energy power relative to total received power.
Once the absolute power Ppilot of the received pilot signal has been ascertained, the path loss between the base station and the mobile unit may be determined using an indication of the power at which the pilot signal was transmitted (Ptransmitted pilot) from the base station. In a conventional CDMA system, in addition to the pilot channel, each cell-site also transmits a setup or “sync” channel. This channel uses the same PN sequence and phase offset as the pilot channel and can be demodulated whenever the pilot channel is being tracked. This sync channel carries, among other things, cell site identification, and the cell site pilot PN carrier phase offset. With this information, the mobile station is capable of establishing system time.
It would be advantageous if the optimum transmitted power levels for link transmitters could be calculated based upon cell-wide, or even more global network considerations.
It would be advantageous to the above-mentioned optimum transmission power levels could be calculated using known received power values, measured at the link receivers.